Injuries are the leading cause of injury into the 3rd decade of life. Alcohol, and dual alcohol and marijuana use are associated with increased risk of injuries. Prior study by this research group (Longabaugh et. Al., 2001) has shown that the combination of a brief motivational interview in the ED followed by a booster session significantly reduces alcohol-related injuries and negative consequences for sub-critically injured harmful/hazardous drinkers more than does either standard treatment or brief intervention by itself. However, hazardous drinkers who also used marijuana had poorer outcomes than did those who did not use marijuana, irrespective of experimental treatment condition. Moreover, Marijuana use was not reduced by the intervention. The implication is that an intervention addressing both alcohol and marijuana use for dual users will be effective in reducing marijuana related injuries and negative consequences as well. The aims of the present study are three: 1) to test whether two sessions of motivational interviewing for dual users will reduce both alcohol and marijuana related injuries and negative consequences more than will standard ED care, 2) to identify the active ingredients of treatment that mediate the effectiveness of the motivational intervention. 3) to identify patient factors that moderate the strength of this relationship between intervention and outcomes]. 600 sub-critically injured dual users will be randomly assigned to motivational treatment vs. standard care, assessed at three months to identify response to treatment, and at one year to measure distal clinical outcomes. Causal chain analyses will be conducted to identify the mediators and moderators of treatment effects. Identification of active ingredients of the treatment will facilitate the generalizeability of the intervention's effects to other ED settings using a brief motivational intervention. [Identification of patient variables that moderate treatment effectiveness will facilitate selection of patients most and least likely to benefit from the intervention.